Flat heat exchanger tubes have been produced from sheet metal strips of limited sheet thickness, with two opposite broad sides and two opposite narrow sides, as well as with at least one connection between the two broad sides dividing the heat exchanger tube into at least two chambers. The connection consists of at least two closely adjacent legs formed by small radius bends along the edges of the strip on one broad side, with the legs forming an angle between them. The feet of the legs are secured on the other broad side.
U.S. Pat. No. 6,209,202 B1 discloses flat heat exchanger tubes of this general type in which quite limited bending radii at the head of the legs are prescribed whereby the closely adjacent legs leave only a very small, roughly triangular hole or gap along the length of the outer periphery of the heat exchanger tube, with the heat exchanger tube being later soldered in the opening to close the tube. U.S. Pat. No. 5,934,365 discloses quite specific small diameters for a circle that fits in this hole or gap, and U.S. Pat. No. 5,890,288 discloses (see particularly FIG. 7 thereof) achieving the small bending radii by applying a perpendicular force to the deformed longitudinal edges by using a tool.
Heat exchanger tubes such as disclosed in these patents require deformation of the edge in specific narrow tolerances, requiring that considerable value must be placed on continuous control of the state of the tool being used (e.g., rollers) and its maintenance.
Further, while it is known from, for example, EP 742 418 B1 (FIG. 2) to achieve narrow bending radii by producing plates using drawing dies which reduce the plates in sheet thickness adjacent to the bending radius, such procedures are difficult to apply to heat exchanger tubes which may be produced by rollers from a sheet metal strip, since flat heat exchanger tubes are frequently no wider in their small dimension (between broad sides) than 2.0 mm and the thicknesses of the sheet used may be in the 0.1 mm range.
The present invention is directed toward overcoming one or more of the problems set forth above.